In addition to injecting heroin directly into a vein, some junkies prefer to inject the drug under the skin. This practice is known as "skin popping." The injection of heroin under the skin does not produce the sudden rush or the same high as mainlining, but there is a longer effect from the drug as it is gradually absorbed into the body from underneath the skin.

The scars that result from skin popping look entirely different than needle tracks. The infection from skin popping first produces round abscesses. The abscesses heal to form characteristic round, shiny scars.